This invention relates generally to electromagnetic flowmeters, and more particularly to a signal generator arrangement adapted to lessen the influence of fluctuations in the flow rate distribution of a fluid passing through a flow tube on the signal electromotive force.
Generally, even if a fluid passing through a flow tube of an electromagnetic flowmeter and intercepting a magnetic field established in the tube undergoes substantial changes in its flow rate distribution, such as a change from the state of a laminar flow to the state of a turbulent flow, this does not give rise to an error in the signal electromotive force induced therein as long as the flow of fluid is symmetrical with respect to the central axis of the flow tube. In the case, however, of an ordinary flow rate distribution wherein fluid flow is not symmetrical with respect to the central axis of the tube, an error is introduced in the signal electromotive force.
To avoid this, the general practice in the prior art is to reduce the error in the signal electromotive force by setting up a magnetic field with a special field distribution, or by making the area of each detecting electrode large relative to the diameter of the flow tube. When a capacitance-detecting type of electromagnetic flowmeter is used to detect the signal electromotive force via a capacitance between the fluid and the detecting electrodes, this meter requires that the electrodes be large. As a consequence, a flowmeter arrangement of this type is not readily affected by asymmetries in the flow rate distribution. These facts are well known in the art.
But as a practical matter, it is not always easy to provide a special magnetic field distribution, and there are other limitations in known expedients for preventing the influence of the flow rate distribution on the induced signal. Moreover, enlargement of the detecting electrode areas introduces difficulties in mounting the electrodes on the flow tube and in sealing off the fluid. There is also the problem of zero drift which is caused by the passage of eddy currents through the detecting electrodes due to a transformer electromotive force generated in the fluid by a magnetic field applied thereto.
Furthermore, in the case of a capacitance-detecting type flowmeter, since the electrostatic capacitance between fluid and the detecting electrode is very small, the output impedance becomes very high. This in turn leads to complexities in the electrode structure and in the detection of the signal electromotive force. Accordingly, the capacitance detecting type flowmeter is not always suitable for the purpose of lessening the influence of the flow rate distribution on the induced signal. Thus the various measures heretofore employed for suppressing the influence of the flow rate distribution on the induced signal have shortcomings of their own.